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In addition to potentially sense vibration through the ground, dinosaurs are believed to have had ears, located on the head, with a frequency sensitivity ranging up to a couple of kHz. Did they have outer ears (pinnae/auricle) to focus sounds and help hearing better in 3D? Pinnae are made of cartilage which does not preserve well in fossils. Their closest still-living relatives, the birds, does not have any pinnae (but some has pinnae-ish made of feathers).

In this SE thread Why are mammals the only animals with pinnae / outer ears?, two related explanations are raised:

  1. (response link) mammal hearing would be so sensitive that any small improvements (such as pinnae) help them to hear better, contrary to other groups where the outer ear improvement would be negligible ;
  2. (response link) there can be a mismatch between the animal body size and the pinna size that would be required to focus sounds for instance mouses and bats have small pinnae which suits their head size but they hear high frequencies (small wavelengths); on the contrary,

Lizards and birds can hear as high as 4kHz to 10 kHz, which corresponds to an energy wavelength of 8cm to 3cm, so a frog [sic] would need ears of about 8cm to collect and focus the sound, and a bird would need ears of about 3cm.

What about large dinosaurs? Their large size could handle large pinnae to hear relatively low frequencies (< some kHz), at least in some species? I'd be curious to know whether the Bioacoustics community has something interesting to say about it, rather than asking it on Biology SE.

enter image description here CC-BY-SA - modified from Valery91Thunder and Kogando

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  • $\begingroup$ Elephants? AFAIK they can hear/feel to <20 Hz, a wave length of ~17 m. If an elephant ear is ~1m they could work from 340 Hz(?), but maybe elephants just move their heads around a bit to make use of phase-locking and inter-aural time difference for low freq sounds? Their ears are maybe mostly for display/temperature regulation? My point is that unless you need very quick directivity information, simply turning your head while the sound is active provides a lot of the same information pinnae would, and for a large head, even quite low frequencies will have a "shadow" behind the head. $\endgroup$
    – Rasmus
    Commented Sep 7, 2022 at 10:42
  • $\begingroup$ @Rasmus Thanks for you thoughts! In addition to spatial hearing, pinnae also focus sound energy to be collected by the inner ear, which can increase sound sensitivity. $\endgroup$
    – Noil
    Commented Sep 8, 2022 at 10:48
  • $\begingroup$ By "relatively low-frequencies", I was thinking of below the kHz range, i.e. ten or tens of cm pinna. I'm gonna edit my post to make it clearer. $\endgroup$
    – Noil
    Commented Sep 8, 2022 at 10:50
  • $\begingroup$ Don't elephants sense infra-sounds through the ground and not via their pinnae? $\endgroup$
    – Noil
    Commented Sep 8, 2022 at 10:53
  • $\begingroup$ Yes that's my understanding, and i bet their sensitivity is poor (or maybe there's a leg-to-inner-ear-link to be discovered ;) ) $\endgroup$
    – Rasmus
    Commented Sep 8, 2022 at 11:19

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This is a good question for a physiologist but much of our understanding of what external structures is based on 1) comparative physiology and 2) the bone structure underlying the muscle groups. As neither birds nor reptiles have pinna it's unlikely, that's the comparative part. For the bone structure, if dinos did have pinna that were movable (like a dog or horses) we might expect them to have more muscular attachment points on the skulls which often look like slightly reinforced plates.

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